It has been a conventional practice to employ nonpolar solvents such as xylene as a solvent for an ink composition for an oil marking pen. In recent years, polar solvents such as alcohols and glycol ethers have been substituted for the nonpolar solvents as the solvent for the ink composition in view of safety.
However, the use of the polar solvents in place of the nonpolar solvents tends to reduce the ink film fixability (the fastness of drawn ink lines such as of letters and illustrations) on a nonabsorbent surface such as a metal surface or a plastic surface, though an excellent ink film fixability is a critical requirement for oil marking pens. This tendency is attributable to the fact that the use of the polar solvents weakens hydrogen bonds between the ink composition and the nonabsorbent surface and an etching effect on the nonabsorbent surface, which may be strong enough to ensure a satisfactory ink film fixability in the case of an ink composition containing a nonpolar solvent, thereby to deteriorate the ink film adherability and fixability on the nonabsorbent surface.
To cope with this problem, the ink film fixability on the nonabsorbent surface is improved by adding a resin or the like to the oil marking pen ink composition containing the polar solvent. For example, Japanese Unexamined Patent Publications No. 3-97773 (1991), No. 3-172366 (1991) and No. 3-250073 (1991) respectively disclose: an ink composition containing a condensation resin of formaldehyde and toluenesulfonamide; an ink composition containing a vinyl acetate resin; and an ink composition containing a polyamide resin and an alcohol-soluble resin (e.g., phenol resin). Further, Japanese Unexamined Patent Publication No. 4-139273 (1992) discloses an ink composition employing, in combination, a resin such as an acrylic resin or a polyamide resin having a hydrophilic group and a resin such as a ketone resin or a rosin-modified maleic acid resin having a hydrophobic group. Japanese Unexamined Patent Publication No. 4-168169 (1992) proposes an ink composition employing, in combination, a resin such as a rosin-modified maleic acid resin or a saturated polyester resin having an acid value of not higher than 10 and a resin such as a ketone resin or a polyamide resin having an acid value of not lower than 80. Japanese Unexamined Patent Publications No. 4-227670 (1992), No. 4-239071 (1992) and No. 4-266975 (1992) respectively disclose: an ink composition containing a polyvinyl alkyl ether and a styrene-acrylic acid copolymer; an ink composition containing a condensation resin of an aldehyde and urea and a cationic surface active agent; and an ink composition containing rosin, a rosin-modified resin, a ketone resin, a methyl ester of hydrogenated rosin, a hydrogenated triethylene glycol ester and the like. Further, Japanese Unexamined Patent Publications No. 5-17717 (1993), No. 5-279616 (1993) and No. 7-118590 (1995) respectively disclose: an ink composition containing an ethanol-soluble ketone resin having a softening point of 60.degree. C. to 100.degree. C. and a phenol resin having a softening point of 60.degree. C. to 100.degree. C.; an ink composition employing a ketone resin, a phenol resin, a rosin resin, a butyral resin and/or a polyamide resin; and an ink composition containing a ketone resin, a phenol resin, a rosin resin, a butyral resin, a polyamide resin and a phthalate.
Even with these ink compositions, the ink film fixability (the fastness of drawn ink lines such as of letters and illustrations) on a nonabsorbent surface such as a metal surface or a plastic surface is unsatisfactory. Therefore, an ink composition having a satisfactory ink film fixability has not been provided yet.